1. Field of the Invention
This invention relates to an automatic adjuster for a shoe drum brake, primarily for motor vehicles, including a device of automatically variable length arranged and operable, in use, in response to wear of the brake shoe linings, to maintain a predetermined maximum shoe to drum clearance. The invention also embraces a shoe drum brake incorporating such an automatic adjuster.
2. Description of the Prior Art
Such adjusters, in various forms, are widely used in motor vehicle braking systems and are generally satisfactory under most conditions of use. However, under certain conditions, such as prolonged braking during a long hill descent, for example, the resultant heating of the drum causes the latter to expand, which has the effect of increasing the shoe to drum clearance and thereby causing the adjuster to produce a compensatory adjustment of the shoes. It is then possible for subsequent contraction of the drum, upon cooling, to bring the drum permanently into contact with the shoes in their newly adjusted retracted positions, thereby locking on the brake. If the adjuster is arranged so as to leave sufficient running clearance to preclude this possibility, then the amount of shoe travel required to enable the shoes to contact the drum during operation at normal temperatures is unacceptably high.
Bi-metallic devices have been used in an attempt to solve this problem and, in one prior proposal applied to an adjuster incorporating a variable length strut between the shoes, a bi-metallic device is disposed between a pair of relatively movable parts of the adjuster strut and sustains any force transmitted through the strut, such as hand brake actuation loads for example. Not only are bi-metallic devices limited in the compensatory effect which they can provide, but in the aforesaid arrangement the bi-metallic device can sometimes be crushed when excessive loads are applied through the strut and for this reason the arrangement can sometimes be unsatisfactory when employed in adjusters in which adjustment takes place while the strut is sustaining a high compressive load.